1. Field of the Invention
The present invention relates to flow measurement systems and more particularly to a flow measurement system for a hemodialysis apparatus having improved fault detection.
2. Description of the Prior Art
Hemodialysis equipment is utilized to remove excess fluids from a patient's blood. Such equipment utilizes a dialyzer having a path through which the blood from the patient flows in and out to be returned to the patient. The blood flow chamber includes a semipermeable membrane through which water and low molecular weight solutes can pass, but not blood. On the opposite side of the membrane, a dialysate chamber is provided through which a dialysate solution flows. The fluids passing through the membrane are picked up by the dialysate which flows out of the dialyzer. The removed fluids are known as filtrates.
To produce the flow of the filtrate through the membrane, there must be a greater pressure in the blood chamber than in the dialysate chamber. The difference in these pressures is referred to as the transmembrane pressure. The operator of the dialysis equipment controls the rate at which filtrate passes through the membrane by increasing or decreasing the transmembrane pressure. With many patients, hypotension and other complications resulting from poor fluid management are a major concern. In such instances, precise control of the volume and flow rate of the filtrate removed from the blood is very important.
To obtain closed control, it is known to use automatic filtration control systems. In such systems, the operator, before the treatment begins, presets the length of time for the treatment and the quantity of fluid to be removed during that time. The automatic controller then measures the elapsed time and the amount of fluid being removed during the process. At any time the fluid removal rate is too small to produce the quantity of fluid required during the remaining time, the controller calculates the rate of filtration needed to reach the fluid removal goal at the end of the treatment and compares that rate with the actual rate at such instant. If the actual rate is too high or too low, the controller will adjust the transmembrane pressure to achieve the rate required to meet the goal.
As will be understood, a controller depends upon having a very accurate measurement of flow of the filtrate through the membrane into the dialysate solution. This measurement can only be made by an accurate measure of the flow rate of dialysate into the dialyzer, an accurate measurement of the flow rate of dialysate plus filtrate out of the dialyzer, and a calculation of the increase in flow rate which would represent the filtrate added to the dialysate. The volume of dialysate passing through the dialyzer is typically greater than the volume of filtrate by a factor of ten or more. Thus, the measurement of volume flow of filtrate requires measurement of a small difference between two larger numbers. This imposes a severe accuracy requirement on the flow transducers.
In hemodialysis equipment manufactured by the assignee of the present application, a flow transducer is disposed at the dialysate input of the dialyzer and a second flow transducer is disposed at the dialysate output of the dialyzer which monitors the dialysate plus the filtrate. A highly accurate flow transducer of the bearingless type, manufactured by Precision Bearingless Flow Company, is used. This type of flow meter utilizes a ring rotor disposed in a vortex chamber having a plurality of symmetrically spaced jets circumferentially around the chamber. A fluid flowing through the jets causes the rotation of the rotor with a rate of rotation proportional to the rate of flow of fluid. The bearingless flow meter provides a plurality of reflective marks on the ring rotor which are monitored by an electrooptical sensor thereby producing an electrical signal whose frequency is proportional to the rate of rotation of the rotor. Once calibrated, these flow meters are specified by the manufacturer as having a 0.001% accuracy. In practice, before beginning treatment with the dialysis equipment, the dialyzer is bypassed, and a uniform flow is produced through the two transducers. The readings of the two flow transducers are compared, and a scale factor calculated to correct any difference in the readings of the two instruments.
Although the bearingless flow transducers satisfy the accuracy requirements, a problem can occur during the treatment of a patient. If the reading of either flow transducer should indicate a change not occasioned by an actual change in flow rates, the automatic filtration controller may accept the erroneous reading as a change in filtration and decrease the transmembrane pressure resulting in inaccuracies of the measurement. Such changes in operation of the bearingless flow transducer have been observed which appear to be caused by slight imbalances in the ring rotors causing an occasional wobble mode of rotation causing the indicated flow rate drops. Therefore, there is a need for a flow rate measurement system which will include means for determining faulty readings of either input or output flow transducers.